Archived posting to the Leica Users Group, 1999/12/01

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Subject: Re: [Leica] RE: 75mm f1.4 DOF
From: "Tom Schofield" <>
Date: Wed, 1 Dec 1999 08:02:03 -0800

I checked AA's book, The Camera.  At p. 49, he explains that DOF  increases
with the square of the change in distance and decreases with the square of
the change in focal length.  Thus, if you move back twice as far and switch
to a lens with twice the focal length (so you will have the same image size
on film) depth of field remains the same.

Last post on this topic, I promise.

Tom Schofield

- ----- Original Message -----
From: Eric Welch <>
To: <>; <>
Sent: Wednesday, December 01, 1999 7:13 AM
Subject: Re: [Leica] RE: 75mm f1.4 DOF

> At 11:03 PM 11/30/1999 -0800, Jacques Bilinski wrote:
> >"  By the "same magnification" I assume you mean the same
> >magnification in the plane of sharpest focus. Say the 50mm is acceptably
> >sharp X feet closer than the plane of sharpest focus, and Y feet beyond
> >plane of acceptable focus. The 100 mm would not have the same
> >as the 50mm at these 2 distances, so it does seem reasonable to me that
> >maybe what matters is some king of 'range in magnification' rather than
> >simply a set distance in front and behind the plane of sharpest focus.
> No, by magnification, what I mean is that a 50mm showing the subject at
> plane of focus of 1/2 life size is going to be X inches from the subject.
> 100mm macro lens at 1/2 life size is going to be 2X the distance. At the
> same aperture, the depth of field will be exactly the same between the two
> lenses. I have talked to many folks who know a lot more about optics than
> that confirm this fact. To say that it is an approximation shows a lack of
> understanding of the nature of optics. Of course, focal length varies
> between two lenses, but not by that much.
> Eric Welch
> Carlsbad, CA
> "There is nothing so stupid as an educated man, if you get off the thing
> that he was educated in. "
>       --Will Rogers